Nafion/Sio2 Hybrid Membrane for Vanadium Redox Flow Battery

Journal of Power Sources 166 (2007) 531–536

Short communication

Naﬁon/SiO2 hybrid membrane for vanadium redox ﬂow battery Jingyu Xi, Zenghua Wu, Xinping Qiu ∗, Liquan Chen Laboratory of Advanced Power Sources, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China Received 15 December 2006; received in revised form 20 January 2007; accepted 24 January 2007 Available online 2 February 2007

Abstract

Sol–gel derived Nafion/SiO2 hybrid membrane is prepared and employed as the separator for vanadium redox flow battery (VRB) to evaluate the vanadium ions permeability and cell performance. Nafion/SiO2 hybrid membrane shows nearly the same ion exchange capacity (IEC) and proton conductivity as pristine Nafion 117 membrane. ICP-AES analysis reveals that Nafion/SiO2 hybrid membrane exhibits dramatically lower vanadium ions permeability compared with Nafion membrane. The VRB with Nafion/SiO2 hybrid membrane presents a higher coulombic and energy efficiencies over the entire range of current densities (10–80 mA cm−2), especially at relative lower current densities (<30 mA cm−2), and a lower self-discharge rate compared with the Nafion system. The performance of VRB with Nafion/SiO2 hybrid membrane can be maintained −2 after more than 100 cycles at a charge–discharge current density of 60 mA cm . The experimental results suggest that the Nafion/SiO2 hybrid membrane approach is a promising strategy to overcome the vanadium ions crossover in VRB. © 2007 Elsevier B.V. All rights reserved.

Keywords: Vanadium redox ﬂow battery; Naﬁon/SiO2 hybrid membrane; Vanadium permeability

1. Introduction Perfluorosulfonic polymers such as Nafion are the most commonly used proton exchange membrane material owning Vanadium redox flow battery (VRB) proposed by Skyllas– to their high proton conductivity, good chemical and ther- Kazacos group [1–4] in 1985 has received considerable atten- mal stability [18]. However, Nafion membrane suffers from tion due to its long cycle life, flexible design, fast response the crossover of methanol and vanadium ions when used in time, deep-discharge capability, and low pollution emitting in direct methanol fuel cell (DMFC) [19,20] and VRB [12,21], energy storage [5–16]. As shown in Fig. 1, the VRB consists of respectively, which results in decreases in energy efficiency. two electrolyte tanks with the electrolytes of V(IV)/V(V) and There has been extensive research activity in the modification V(II)/V(III) in sulfuric acid solution, two pumps, and a battery of Nafion based membrane to reduce the methanol permeability, stack section where the redox electrode reaction takes place. for example, recasting with inorganic nanoparticles, [22] surface The electrolytes are pumped into the stack separated by an ion modification with metal thin film, [23] surface modification with exchange membrane. Ion exchange membrane is one of the key layer-by-layer self-assembly polyelectrolyte, [19] hot-pressing materials for VRB and usually used to provide proton conduction with other proton conducting membrane, [24] and in situ sol–gel to maintain the electrical balance and effective separation of the reaction to incorporate inorganic oxide nanoparticles within the anode and cathode electrolytes. The ideal membrane for VRB pores of Nafion [25]. All of above modification methods may should possess low vanadium ion crossover, high ionic conduc- also be used to decrease the crossover of vanadium ions through tivity, and good chemical stability. Previous studies showed that Nafion membrane and, in general, improves the performance of most early types of commercial ion exchange membranes (e.g., VRB. Selemion CMV, DMV, Asahi Glass Co., Japan) are unsuitable Since the pioneer work by Mauritz et al. [26–29], sol–gel due to their degradation by V(V) in VRB [17]. derived Nafion/SiO2 hybrid membranes have been successfully used in DMFC due to the simple dealing procedure (see Fig. 2) and lower methanol permeability. Nafion/SiO2 hybrid mem- ∗ Corresponding author. Tel.: +86 755 26036879; fax: +86 755 26036879. branes can reduce the crossover of methanol because of the E-mail addresses: [email protected] (X. Qiu), polar clusters (pores) of the origin Nafion, which is the dominat- [email protected] (J. Xi). ing reason for methanol permeating, had been filled with SiO2

nanoparticles during the in situ sol–gel reaction with TEOS, as shown in Fig. 2 [25,29]. The specific nanostructure of Nafion/SiO2 hybrid membranes inspired us that this kind of membrane maybe also can lower vanadium ion permeability compared with Nafion membrane. In this work, Nafion/SiO2 hybrid membrane was measured their ion exchange capacity (IEC), proton conductivity, water uptake, and vanadium ion permeability in comparison with the pristine Nafion 117 membrane. Then, Nafion/SiO2 hybrid membrane was employed as the separator in VRB, and the cell performance was also evaluated and discussed. Experiment results show that the Nafion/SiO2 hybrid membrane approach is a promising strategy to inhibit vanadium ions crossover in VRB.

2. Experimental

2.1. Preparation of Naﬁon/SiO2 hybrid membrane

All membranes used in this work were Nafion 117 and denoted as Nafion. Prior to modification, the Nafion membrane was treated according to the standard procedure that is 60 min ◦ in a 3 wt.% H2O2 solution at 80 C, 30 min in deionized water at Fig. 1. Schematic illustration of vanadium redox flow battery (VRB). ◦ −1 ◦ 80 C, and 30 min in 1 mol L H2SO4 solution at 80 C. After each treatment, the membrane was rinsed in deionized water to remove traces of H2O2 and H2SO4. The membrane was stored in deionized water before use. Samples were dried at 110 ◦C under vacuum to determine the initial, dry H+ form weight before the sol–gel reaction. Nafion/SiO2 hybrid membrane was prepared according to the in situ sol–gel method reported by Mauritz et al. [26–29] Fig. 2 illustrates the preparation of Nafion/SiO2 hybrid membrane [25,29]. In a typical synthesis, a 6 cm × 6 cm dry Nafion membrane was first swollen overnight in stirred solutions of MeOH:H2O = 5:1 (vol/vol) at room temperature. Then the pre- mixed tetraethylorthosilicate (TEOS)/MeOH solutions were introduced into the flask with the Nafion membrane so that the H2O:TEOS ratio was 4:1 (mol/mol) while maintaining stirring. After 3 min for the sol–gel reaction, the membrane was removed from the flask, and then quickly soaked in MeOH for 1–2 s to wash away excess reactants adhering to the surface. Finally, the membrane was surface-blotted and dried at 100 ◦C under vacuum for 24 h. The resulted Nafion/SiO2 hybrid membrane had a silica content of 9.2 wt.% (see Table 1). The sample was stored in deionized water before use.

2.2. Membrane characterization

The water uptake of the membranes was deﬁned as mass Fig. 2. Schematic depiction of the preparation of Naﬁon/SiO2 hybrid membrane. ratio of the absorbed water to that of the dry membrane. It can

Table 1 Comparison of general properties between Naﬁon and Naﬁon/SiO2 hybrid membrane Membrane Silica content (wt.%) Thickness (␮m) Water uptake (wt.%) IEC (mmol g−1) Conductivity (mS cm−1)

Naﬁon – 215 26.0 0.97 58.7 Naﬁon/SiO2 9.2 204 21.5 0.96 56.2 J. Xi et al. / Journal of Power Sources 166 (2007) 531–536 533

Fig. 3. (a) Schematic illustration of the cell for the measurement of vanadium permeability. (b) Vanadium ion concentration in the right reservoir of the cell with Nafion (᭹) and Nafion/SiO2 () membranes. (c) Comparison of the permeability of vanadium ions through the membrane between Nafion and Nafion/SiO2 hybrid membrane. be calculated using the equation: was 5.0 cm2 and the volume of the solution for each reservoir was 20 mL. Samples of solution from the right reservoir were (W − W ) = w d × taken at a regular time interval and analyzed for vanadium ion water up take W 100% d concentration by inductively coupled plasma atomic emission spectrometry (ICP-AES). where Ww is the weight of the wet membrane, and Wd is the weight of the dry membrane. The IEC of the membranes was measured by the method described previously [30]. 2.3. Cell tests Proton conductivity of the membranes was determined by measuring the impedance spectroscopy on a cell with the given The VRB used in the charge–discharge tests was fabricated membrane sample sandwiched between two stainless steel (SS) by sandwiching the membrane (6cm × 6cm) between two pieces electrodes. The measurements were carried out on a Solartron of carbon felt (Shanghai Xinxing Carbon Materials Co., Ltd., 1255 B frequency response analyzer coupled with a Solartron thickness is 5 mm) electrodes and then clamping the sandwich 1287 electrochemical interface in the frequency range of 1 Hz between two graphite polar plates which were graved with to 1 MHz at 30 ◦C and 100% related humidity. The conductivity serpentine flow fields. The area of the electrode for the reac- was calculated according to the electrode area of the cell and tion was 25 cm2. At the beginning of charge–discharge cycles, −1 −1 the thickness of the membrane, which was measured with a 40 mL of 2 mol L V(IV) in 2.5 mol L H2SO4 solution was − micrometer. pumped into the cathode side and 40 mL of 2 mol L 1 V(III) in −1 Fig. 3(a) illustrates the equipment used for the measure- 2.5 mol L H2SO4 solution was pumped into the anode side, ment of the permeability of three types of vanadium ions [12]. respectively. To avoid the corrosion of the carbon felt electrode The V(IV) solution was prepared by dissolving VOSO4·5H2O and graphite polar plates, the cell was charged to 1600 mA h −1 (Shanghai XinYue Co., Ltd.) in 2.5 mol L H2SO4. V(III) and with a corresponding redox couples utilization of 75%. The low V(V) solutions were prepared by the electrochemical reduction voltage limit for discharge was controlled to 0.8 V. and oxidation of the V(IV) solution, respectively [31]. The left reservoir was filled with 1 mol L−1 V(III), V(IV), or V(V) ion 3. Results and discussion −1 solution in 2.5 mol L H2SO4, and the right reservoir was filled −1 −1 with 1 mol L MgSO4 solution in 2.5 mol L H2SO4. MgSO4 3.1. Physical properties was used to equalize the ionic strengths of the two solutions and to minimize the osmotic pressure effect [14]. The two solutions Mauritz et al. have reported that during the preparation pro- were continuously stirred using magnetic stir bars during exper- cess of sol–gel derived Nafion/SiO2 hybrid, the TEOS molecules iments at room temperature. The two solutions were separated will preferentially migrate to the polar clusters inside the Nafion by a membrane. Before use, the membrane was immersed in membrane and the subsequently hydrolysis of sorbed TEOS distilled water. The geometrical area of the exposed membrane are confined to these clusters to form SiO2 nanoparticles filled 534 J. Xi et al. / Journal of Power Sources 166 (2007) 531–536

Nafion/SiO2 hybrid membrane, as shown in Fig. 2 [29]. Some properties of the Nafion membrane and Nafion/SiO2 hybrid membrane are summarized in Table 1. It can be seen that Nafion/SiO2 hybrid membrane has nearly the same IEC and proton conductivity as pristine Nafion membrane. The slight decrease in water uptake for Nafion/SiO2 hybrid membrane can be attributed to the filling of polar clusters by SiO2 nanoparticles, as shown in Fig. 2.

3.2. Permeability of vanadium ions

The permeability of three types of vanadium ions, i.e. V(III), V(IV) and V(V), through the membrane was measured under the same condition by using the equipment shown in Fig. 3(a). In this work, we did not measure the permeability of the V(II) ion across the membranes since the V2+ ion is easily oxidized in the air. Fig. 3(b) shows the relationships of the concentration of vanadium ions in the right reservoir with time. It can be seen that the concentration of vanadium ions in the right reservoir for Nafion membrane increased faster than that for Nafion/SiO2 hybrid membrane. It is supposed that the change in vanadium ion concentration in the left side can always be negligible during the calculation of permeability due to the fact that the concentration of the vanadium ion in the right reservoir is low. Inside the membrane, a pseudo-steady-state condition is used. Accordingly, the flux of the vanadium ion is constant, and its concentration in the right reservoir as a function of time is given by the equation [12,32]:

c t P Fig. 4. Charge–discharge curves for VRB with Nafion membrane (a) and V d R( ) = A c − c t R t L [ L R( )] Nafion/SiO2 hybrid membrane (b) at various current densities. Charge capacity d was controlled to be 1600 mA h corresponding to a redox couples utilization of 75%. where cL is the vanadium ion concentration in the left reservoir, and cR(t) refers to the vanadium ion concentration in the right reservoir as a function of time. A and L are the area and discharge curves of VRB with Nafion and Nafion/SiO2 hybrid thickness of the membrane, respectively. P is permeability of the membranes, it can be seen that the discharge capacity of the vanadium ions, and VR is the volume of right reservoir, respec- VRB with Nafion/SiO2 hybrid membrane is higher than that of tively. An assumption is also made here that P is independent of the VRB with Nafion membrane at all charge–discharge current concentration. densities, which indicates that Nafion/SiO2 hybrid membrane The vanadium ion permeability P inside the membrane is has excellent performance to restrain the crossover of vanadium calculated and compared in Fig. 3(c). Among the three vana- ions. As expected, the discharge capacity increases with the dium ions, the V(III) ion has the highest permeability through charge–discharge current density increasing. This is attributed the membranes and the V(V) ion has the lowest, which may be to the shorter charge–discharge time at high current densities; attributed to the different diffusion coefficients and the charges therefore, the amount of vanadium ions that cross over the mem- of these vanadium ions when they are diffusing in a cation branes will be reduced. exchange membrane. It is exciting to note that Nafion/SiO2 The relationships of the coulombic efficiency (CE), hybrid membrane has dramatically lower vanadium ion perme- voltage efficiency (VE) and energy efficiency (EE) with ability compared to that of the Nafion membrane, especially charge–discharge current density are illustrated in Fig. 5.As for high valence state vanadium ion. It can be estimated that the shown, the CE of VRB increases with the increasing of current VRB using the Nafion/SiO2 hybrid membrane will have a higher density, while the VE shows the completely opposite trend. For coulombic efficiency (CE) than the one using pristine Nafion all current densities and especially at lower current densities −2 membrane for its lower permeation rate of vanadium ions. (<30 mA cm ), the VRB with Nafion/SiO2 hybrid membrane shows higher CE and EE than that of Nafion system. It shows 3.3. VRB performance a maximum EE of the VRB, which is 79.9% for Nafion/SiO2 hybrid membrane and 73.8% for the Nafion membrane both at Fig. 4 displays the charge–discharge plots of a VRB single 20 mA cm−2. The differences of the CE and EE between the cell at different current densities. By comparing the charge– VRB with Nafion and with Nafion/SiO2 hybrid membranes are J. Xi et al. / Journal of Power Sources 166 (2007) 531–536 535

Fig. 7. Cycle performance for VRB with Naﬁon/SiO2 membrane. Charge capacity was controlled to be 1600 mA h corresponding to a redox couples utilization of 75%.

nanoparticles [25–29], and hence, a relatively high CE and EE can be achieved. Open circuit voltage (OCV) of the VRB was monitored at room temperature after it was charged to a 75% state of charge (SOC) and is presented in Fig. 6. The electrolytes solutions were pumped through the cell unceasingly during the self-discharge test. As illustrated, the OCV value gradually decreases with storage time at first and then rapidly drops to ca. 0.75 V.From Fig. 6, it can also be seen that the maintaining time of OCV above Fig. 5. Effect of current density on coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of the VRB with Nafion and Nafion/SiO2 1.0 V of VRB with Nafion/SiO2 hybrid membrane is about 35 h, membrane, respectively. which is nearly two times longer than that of VRB with pristine Nafion membrane. This means that the rate of self-discharge of also partly attributed to the difference of vanadium ions per- VRB with Nafion is faster than that of VRB with Nafion/SiO2 meability through membranes. In the case of VRB with Nafion hybrid membrane. The self-discharge of VRB is mainly due to membrane, vanadium ions will crossover through the polar clus- the crossover of vanadium ions through the membrane between ters (pore) of Nafion membrane during the charge–discharge the cathode reservoir and anode reservoir, i.e., V(II) and V(III) cycles and react with those vanadium ions which have different from anode to cathode and V(IV) and V(V) from cathode to valence state, resulting in a lower CE and EE. On the contrary, anode. for the VRB with Nafion/SiO2 hybrid membrane, the transport- The cycle performance of a VRB single cell employing the ing of vanadium ions through the membrane has been greatly Nafion/SiO2 hybrid membrane at a charge–discharge current −2 reduced due to the filling of polar clusters of Nafion by SiO2 density of 60 mA cm is presented in Fig. 7. As shown, there is nearly no decay of CE and EE up to 100 cycles. It means that Nafion/SiO2 hybrid membrane possesses high stability in vanadium solutions at strong acid condition and thus is able to maintain the cell performance.

4. Conclusions

Nafion/SiO2 hybrid membrane was prepared by in situ sol–gel method and shows nearly the same IEC and proton conductivity as that of pristine Nafion 117 membrane. Nafion/SiO2 hybrid membrane displays very lower vanadium ions permeability compared to Nafion membrane, for its polar clusters is filled with SiO2 nanoparticles. The VRB single cell with Nafion/SiO2 hybrid membrane shows a higher coulombic and energy efficiencies and a lower self-discharge rate than that Fig. 6. Open circuit voltage (OCV) of the VRB as a function of time at the state of Nafion system does. The results from cycling tests reveal of charge (SOC) of 75%. that Nafion/SiO2 hybrid membrane has good chemical stabil- 536 J. Xi et al. / Journal of Power Sources 166 (2007) 531–536 ity in vanadium and acid solutions. All experimental results [10] C.J. Rydh, J. Power Sources 80 (1999) 21. confirm that the Nafion/SiO2 hybrid membrane approach is a [11] H. Vafiadis, M. Skyllas-Kazacos, J. Membr. Sci. 279 (2006) 394. promising strategy to overcome the vanadium ions crossover [12] X. Luo, Z. Lu, J. Xi, Z. Wu, W. Zhu, L. Chen, X. Qiu, J. Phys. Chem. B 109 (2005) 20310. in VRB. We believe that by optimizing the SiO2 content in [13] G. Oriji, Y. Katayama, T. Miura, J. Power Sources 139 (2005) 321. Nafion/SiO2 hybrid membrane or by using organically modi- [14] T. Sukkar, M. Skyllas-Kazacos, J. Appl. Electrochem. 34 (2004) 137. fied silicate (ORMSIL) to replace SiO2, the performance of the [15] B. Tian, C.W. Yan, F.H. Wang, J. Membr. Sci. 234 (2004) 51. VRB can be further improved. These works are now underway [16] B. Fang, Y. Wei, T. Arai, S. Iwasa, M. Kumagai, J. Appl. Electrochem. 33 in our lab and will be reported in the near future. (2003) 197. [17] T. Mohammadi, M. Skyllas-Kazacos, J. Appl. Electrochem. 27 (1996) 153. Acknowledgment [18] K.A. Mauritz, R.B. Moore, Chem. Rev. 104 (2004) 4535. [19] S.P. Jiang, Z.C. Liu, Z.Q. Tian, Adv. Mater. 18 (2006) 1068. The authors appreciate financial support from the National [20] M.A. Hickner, H. Ghassemi, Y.S. Kim, B.R. Einsla, J.E. McGrath, Chem. Natural Science Foundation of China (50606021). Rev. 104 (2004) 4587. [21] G.-J. Hwang, H. Ohya, J. Membr. Sci. 132 (1997) 55. [22] C.H. Rhee, H.K. Kim, H. Chang, J.S. Lee, Chem. Mater. 17 (2005) 1691. References [23] J. Prabhuram, T.S. Zhao, Z.X. Liang, H. Yang, C.W. Wong, J. Electrochem. Soc. 152 (2005) A1390. [1] E. Sum, M. Skyllas-Kazacos, J. Power Sources 15 (1985) 179. [24] B. Yang, A. Manthiram, Electrochem. Commun. 6 (2004) 231. [2] E. Sum, M. Rychcik, M. Skyllas-Kazacos, J. Power Sources 16 (1985) 85. [25] N. Miyake, J.S. Wainright, R.F. Savinell, J. Electrochem. Soc. 148 (2001) [3] M. Skyllas-Kazacos, M. Rychcik, R. Robins, J. Electrochem. Soc. 133 A898. (1986) 1057. [26] K.A. Mauritz, R.M. Warren, Macromolecules 22 (1989) 1730. [4] M. Skyllas-Kazacos, M. Rychcik, R. Robins, All-vanadium redox battery, [27] K.A. Mauritz, I.D. Stefanithis, Macromolecules 23 (1990) 1380. US Patent 4786567 (1988). [28] K.A. Mauritz, I.D. Stefanithis, S.V. Davis, R.W. Scheetz, R.K. Pope, G.L. [5] C. Ponce de Leon, A. Frıas-Ferrer, J. Gonzalez-Garcıa, D.A. Szanto, F.C. Wilkes, H.H. Huang, J. Appl. Polym. Sci. 55 (1995) 181. Walsh, J. Power Sources 160 (2006) 716. [29] Q. Deng, R.B. Moore, K.A. Mauritz, J. Appl. Polym. Sci. 68 (1998) 747. [6] C.J. Rydh, B.A. Sanden, Energy Convers. Manage. 46 (2005) 1957. [30] G.-J. Hwang, H. Ohya, J. Membr. Sci. 120 (1996) 55. [7] C.J. Rydh, B.A. Sanden, Energy Convers. Manage. 46 (2005) 1980. [31] F. Grossmith, P. Liewellyn, A.G. Fane, M. Skyllas-kazacos, Proceedings [8] L. Joerissen, J. Garche, Ch. Fabjan, G. Tomazic, J. Power Sources 127 of the Symposium of the Electrochemical Society, vol. 88-11, The Electro- (2004) 98. chemical Society, Honolulu, HI, October, 1998, p. 363. [9] Ch. Fabjan, J. Garche, B. Harrer, L. Jorissen, C. Kolbeck, F. Philipps, G. [32] X. Qiu, W. Li, S. Zhang, H. Liang, W. Zhu, J. Electrochem. Soc. 150 (2003) Tomazic, F. Wagner, Electrochim. Acta 47 (2001) 825. A917.